Hey Jim,
The 737 and 107 are similar, but not exactly the same. The biggest difference is the G107d's PTO turns the opposite direction in front mount mode. The Grillo will run any rear mount attachment the 737 will run, but it can only run the sickle mower in front mount mode. Many attachments are designed to be reversible (you can flip the gearbox on both the sweeper and the snowblower), but some (like the BCS mowers) are not.
I never thought of putting a flat face coupler on a Grillo, but i don't see why it couldn't be done. The PTO on the 737 and the 107d are very similar.
Grillo's tiller design is definitely superior to BCS's design in many ways - bolo tines versus straight tines; helical pattern versus straight pattern; easier-to-change tines versus the nightmare that is BCS's tine mounting (you can't even get the right tines for an older tiller from BCS); 6 width options in one tiller versus 2 options; slimmer, more robust gearbox versus the same gearbox BCS made since the 1980's, easier-to-change, more effective depth control versus BCS's two-hands-and-a-shoulder, less effective depth control; etc. All that AND it's considerably less expensive - $550 for a Grillo 17, 18, 20, 22, 25, 27 inch versus $650 for a BCS 20, 26 inch or $700 for a BCS 26, 30 inch tiller.
